FWIW, I've usually just changed the pads after doing a quick measure of thickness on the rotor to be safe. Not so much on my MB, but on my mazda 929 I've changed pads 3 times in 140K miles and the brakes continue to work fine even thought the rotors are a LITTLE grooved. No shaking or pedal pulse and I don't let the pads get too thin before swapping them out. I just changed the MB pads a while back and basically did the same thing and they seem OK. I think if the rotors are only a little grooved you can just let it go at that as long as they're more or less in thickness spec. Measure in a few spots before decideing if the rotors are OK.